The invention relates to a method for controlling the injection, by a reductant injector in an exhaust system for an internal combustion engine, of reductant for an exhaust aftertreatment unit of the exhaust system, for example a selective catalytic reduction (SCR) unit. The invention also relates to a computer program, a computer readable medium, a controller, and a vehicle.
Selective catalytic reduction (SCR) is known for reducing nitrogen oxides (NOx) in emissions from internal combustion engines, for example diesel engines used in heavy vehicles such as trucks or buses. SCR basically converts NOx with the aid of a catalyst into nitrogen and water. SCR involves adding a reductant, e.g. a gaseous reduction, such as anhydrous ammonia, aqueous ammonia or urea, to the exhaust gas, whereby the reductant is adsorbed onto the catalyst, presenting a carrier and an active catalytic component. Such reductant may be known under names like AdBlue or DEF. It should be noted that where urea is used, it is usually convened to ammonia before being absorbed.
A challenge in SCR operation is to provide for a reductant injection flow which is adapted to the current operating condition of the engine and the exhaust system. The result can be inadequate reduction of NOx. Suggestions to improve the control of SCR operation have been made. EP1164266A2 discloses determining the NOx, conversion and the ammonia concentration downstream of an SCR unit, and using these for control of the amount of ammonia supplied to the SCR. US2011283678 discloses switching between closed and open loop urea controlling procedures based on a SCR influencing parameter. US20120067028 describes determining an SCR degradation value used in control of the supply of reductant.
However, with increasingly higher demands on exhaust aftertreatment, there is still a desire to further improve the control of SCR.
It is desirable to improve the control of the injection of reductant for an exhaust aftertreatment unit in a vehicle. It is also desirable to secure adequate reduction of NOx in an exhaust aftertreatment unit in a vehicle.
According to an aspect of the invention, a method is provided for controlling the injection, by a reductant injector in an exhaust system for an internal combustion engine, of reductant for an exhaust aftertreatment unit of the exhaust system, for example a selective catalytic reduction (SCR) unit, characterised in
determining a reductant injection debt in dependence on a reductant flow according to at least a first request being higher than a threshold of the reductant flow, and
at least partly in dependence on the reductant injection debt, controlling the reductant injector so as to inject a compensation flow.
The invention is particularly advantageous where the threshold of the reductant flow is a maximum flow of a reductant dosing system comprising the reductant injector. The invention provides a parameter in the form of the reductant injection debt, which can be adjusted upon an indication on the reductant flow according to at least the first request being higher than a maximum reductant flow limit. The invention is based on the realisation that reductant requests exceeding the capacity of the reductam injector can provide inaccuracies in the control, of the exhaust aftertreatment unit. The problem normally occurs at, high engine loads where the NOx flow is normally high. The problem is particularly pronounced in connection with engines with very high engine out NOx flows, having exhaust systems with so called extended SCR variants. The problem is also especially high where exhaust system controllers are adapted to compensate for reductant diluted with water, which is done by increasing the reductant injection flow. Reductant requests exceeding the capacity of the reductant dosing system may result in increased system out NOx.
The invention provides, through the reductant injection debt and the compensation flow, for requested reductant which has not been injected, for example due to hardware restrictions of the reductant dosing system, to be injected through a debt repayment as soon as lower requested reductant injection flows allow it, e.g. in view of hardware restrictions. This reduces the system out NOx impact in particular at transient engine operations. Since the invention allows for the demanded reductant to be injected as soon as possible, it allows the total mass of finally injected reductant to be approximately the same as the demanded reductant mass.
Besides better SCR efficiency average in transient operations, advantages of the invention include a reduced need, due to the better SCR efficiency, for controlling the engine to provide a low engine out NOx level. Such low engine out NOx control increases fuel consumption and engine out soot emissions. Thus, the reduced need for low engine out NOx control will reduce fuel consumption and soot emissions. Also, the improved reductant injection control of the invention provides for an increased margin to an inducement trigger threshold for redact dilution tampering.
It should be noted that the reductant injection debt could be provided in any suitable unit, e.g. expressed in volume, or mass. The reductant flow can be provided in any suitable unit. e.g. expressed in volume flow, or mass flow. It should be noted that the reductant flow according to at least the first request could be a single flow based on one or several requests, or a number of flows, each according to a respective request. Preferably, the reductant injection debt is determined in dependence on a reductant flow according to a first request being higher than a threshold of the reductant flow.
Preferably, the reductant injector is controlled so as to inject the compensation flow in addition to a subsequent injection of a reductant flow according to a second request. Preferably, the reductant flow according to the second request is below the threshold of the reductant flow. Thereby, where subsequently to the situation, with a requested reductant flow being higher than the maximum reductant flow, a situation is identified in which the requested reductant flow is less than the maximum reductant flow, the compensation flow is injected and thereby the reductant debt can be “repaid”. The situation in which the requested reductant flow is less than the maximum reductant flow may be identified when it occurs, or by a predictive operation based on values of relevant engine and/or exhaust system parameters.
Preferably, determining the reductant injection debt comprises determining the reductant injection debt at least partly based on the excess of the reductant flow according to the first request in relation to the threshold of the reductant flow. Preferably, the step of determining the reductant injection debt comprises determining whether to increment the reductant injection debt. Preferably, determining the reductant injection debt comprises increasing the reductant injection debt if the reductant flow according, to the first request is higher than the threshold of the reductant flow. Thereby, the adjustment of the reductant injection debt is related to the surplus of the reductant flow according to the first request in relation to the threshold of the reductant flow. This provides for the reductant debt may to accurately reflect omitted injections due to the injection system restrictions.
Preferably, the step of controlling so as to inject the compensation flow is dependent on a decision on whether to inject the compensation flow or not. Thereby the compensation flow injection can be directly dependent not only on the reductant injection debt, but also on other parameters and conditions, which may advantageously be taken into consideration for the reductant flow control.
Preferably, the compensation flow is partly dependent on the size of the reductant injection debt. For example, depending on the size of the reductant injection debt, the calculation of the compensation flow can be adjusted. E.g., the compensation flow can be set to zero if the reductant debt is zero, or the size of a non-zero reductant debt can be decisive for the compensation flow size.
Preferably, the method comprises determining an age of the reductant injection debt, wherein the control so as to inject the compensation flow is partly dependent on the determined debt age. Thereby, when compensating for non-injected portions of requested reductant injections, the compensation flow might be reduced due to the debt age. Thereby consideration may be taken to the fact that where the debt age is relatively high, benefits of a full compensation flow might be less pronounced. Determining the debt age may involve storing, when recording in a data storage an increment of the reductant injection debt, an entry of the time of the recording. The stored data of the reductant injection debt may thereby include a number of registrations of respective points in time of respective debt portions corresponding to respective debt increments. In some embodiments, the debt age is determined as the average age of the debt increments in the reductant injection debt. In other embodiments, the debt age is determined as the age of the oldest debt increment in the reductant injection debt. The debt age might be determined any suitable manner, even as the age of the most recent debt increment in the reductant injection debt.
Preferably, the method comprises determining an operational condition of the internal combustion engine or the exhaust system, wherein the control so as to inject the compensation flow is partly dependent on the determined operational condition. Thereby, the compensation flow might be reduced due to the operational condition. Thereby consideration may be taken to the fact that under some operational conditions, a reduced compensation flow might provide the same benefits as a full compensation flow, or even provide improved benefits.
Preferably, the method comprises determining a temperature in the exhaust system, for example in the exhaust aftertreatment unit, wherein the control so as to inject the compensation flow is partly dependent on the determined temperature. Thereby, the control so as to inject the compensation flow can be partly on dependent on a rate of change of said temperature. Thereby consideration may be taken to the fact that the benefits provided by the compensation flow will be dependent on the temperature or the rate of change of the temperature of the exhaust aftertreatment unit.
Preferably, the method comprises determining a condition of a buffer of reductant exhaust aftertreatment unit of in the exhaust system, wherein the control so as to inject the compensation flow is partly dependent on the determined condition of the buffer of reductant. Thereby consideration may be taken to the fact that the benefits provided by the compensation flow will be dependent on the reductant buffer of the exhaust aftertreatment unit. For example, where there is, a relatively high amount of reductant in the reductant buffer, the benefits of a reduced compensation flow might be the same or higher than those of a full compensation flow.
Preferably, the method comprises determining an amount of nitrogen oxides in exhaust gases in the exhaust system, wherein the control so as to inject the compensation flow is partly dependent on the determined amount of nitrogen oxides. Thereby consideration may be taken to the fact that the benefits provided by the compensation flow will be dependent on the amount of nitrogen oxides in exhaust, gases. For example, where there is a relatively low amount of nitrogen oxides in exhaust gases, the benefits of a reduced compensation flow might be the same or higher than those of a full compensation flow.
Preferably, the method comprises determining a decrease of the reductant injection debt at least partly based on the compensation flow. Thereby, the reductant injection debt may be reduced as the debt is “repaid” by the compensation flow.
Preferably, the method comprises determining a decrease of the reductant injection debt at least partly based on the size of the reductant injection debt. Thereby consideration may be taken to the fact that at relatively large reductant injection debts, reduction of the full debt solely by injection of a compensation flow may not provide any benefits in relation to the injection of a compensation flow for full reduction of a smaller debt. A reason for this might be that the “repayment” of a large debt will take a relatively long time, during which the effects sought by the injection requests made when the debt was build up, will not be provided anymore, e.g. due to changed operational circumstances.
Preferably, the method comprises determining an age of the reductant injection debt, and determining a decrease of the reductant injection debt at least partly based on the age of the reductant injection debt. Thereby consideration may be taken to the fact that where the debt age is relatively high, benefits of a full compensation flow might be less pronounced.
Preferably, the method comprises determining an operational condition of the internal combustion engine or the exhaust system, and determining a decrease of the reductant injection debt at least partly based on the determined operational condition. Thereby an advantageous feature is provided since a full “repayment” of the debt might under some operational conditions provide the same or even less beneficial results as a “repayment” of a reduced debt. Thereby, the injection of an unnecessary surplus of reductant is avoided.
Preferably, the method comprises determining a temperature in the exhaust system, for example in the exhaust aftertreatment unit, and determining a decrease of the reductant injection debt at least partly based on the determined temperature. This is advantageous since a repayment of a full debt might at some exhaust system temperatures provide the same or even less beneficial results as a repayment of a reduced debt. Thereby, the injection of an unnecessary surplus of reductant is avoided.
Preferably, the method comprises determining a condition of a buffer of reductant in the exhaust aftertreatment unit, and determining a decrease of the reductant injection debt at least partly based on the condition of the buffer of reductant. Thereby, the reductant injection debt may be “forgotten” if no emission benefit to inject the reductant injection debt can be reached due to the reductant buffer situation. Alternatively, the reductant injection debt may be partly reduced due to the reductant buffer situation. Thereby, the injection of an unnecessary surplus of reductant is avoided.
Preferably, the method comprises determining an amount of nitrogen oxides in exhaust gases in the exhaust system, and determining a decrease of the reductant injection debt at least partly based on the determined amount of nitrogen oxides. This is advantageous since a repayment of a full debt might at some exhaust gas nitrogen oxide concentrations provide the same or even less beneficial results as a repayment of a reduced debt. Thereby, the injection of an unnecessary surplus of reductant is avoided.
Determining a decrease of the reductant injection debt may comprise determining a rate of decrease of the reductant injection debt. Thereby an advantageous manner of adjusting the debt, e.g. due to the operational situation of the exhaust system, is provided.
Preferably, the method comprises determining a first torque of the engine, wherein the first requested reductant flow is determined at least partly based on the first engine torque. Preferably, the method comprises determining a second torque of the engine, wherein the second requested reductant flow is determined at least partly based on the second engine torque.